Information recording-reproducing units for recording and/or reproducing video and audio information on a rotational recording medium such as FDs (flexible disks), CDs (compact disks), DVDs (digital versatile disks), MDs (mini-disks), and HDs (hard disks), have been widely used. In order to control recording and reproduction of information correctly on a disk, it is necessary for such recording-reproducing unit to have a device for detecting the initial and final rotational positions of the disk associated with the information.
Conventionally, the rotational potion of the disk is detected based on an index signal, which is generated from a signal (rotor rotation signal) indicative of the rotation of the rotor of a motor rotating the disk once for every rotation of the disk. For example, a rotational position detector for use with a conventional FDD system generates a pulsed index signal for each rotation of an FD.
FIG. 11 shows a brushless motor having a conventional rotational position detector. This rotational position detector includes: a rotor having a multiplicity (8 in the example shown) of drive magnetic poles 11; a stator having Hall elements 2 for detecting the magnetic fluxes of the drive magnetic poles 11 to generate 3-phase Hall output signals HU, HV, and HW for controlling driving of the motor, and 3-phase drive coils 13; and a brushless motor drive circuit 15 for switching drive current to the 3-phase drive coils 13 in response to a drive magnetic pole position signal. The motor shown in FIG. 11 is equipped with a sensor for controlling driving of the motor using the Hall signals HU, HV, and HW.
In this rotational position detector, one of the drive magnetic poles of the rotor (the pole referred to as weak magnetic pole) has a non-magnetic region 12 to reduce the magnetic flux of that pole as compared with other the magnetic poles. Thus, the amplitude of the output signal generated by a smaller magnetic flux in the Hall element 2 facing the weak magnetic pole is smaller than that generated by other poles. This can be utilized to generate an index signal.
Of the 3-phase Hall signals HU, HV, and HW, the Hall signal HW associated with W phase is used by the rotational position detector of FIG. 11 to generate the index signal. The Hall signal HW is supplied to an amplifier circuit 3 to generate an amplified Hall output signal Vhw.
The maximum level, i.e. peak value, of the Hall signal Vhw is held in a peak hold circuit 4. A reference signal Vr having a predetermined magnitude relative to the peak value Vhwp is generated by a reference voltage generation circuit 5. A voltage comparison circuit 6 compares the Hall output signal Vhw with the reference signal Vr to generate a comparison output in association with the weak magnetic pole. Based on the comparison output and the signal output from a waveform shaping circuit 16 waveform shaping the Hall output signal Vhw, a discrimination circuit 17 generates one index signal for each rotation of the disk (reference 1: JP-2569736).
In recent years, in an effort to make FDD and HDD less influenced by design limitations on the dimensions (in height and area) thereof, and to reduce in number and in cost power lines, lead wires, and Hall elements in a brushless motor, sensor-less brushless motors utilizing no such 3-phase Hall elements have been increasingly used.
These sensor-less motors also require at least one Hall element to generate an index signal, since it is still necessary for these motors used in information recording-reproducing units to provide an index signal in association with the rotation of the disk. However, provision of a Hall element in the sensor-less motor solely for generating the index signal still presents a design limitation and cost problem to the motor.